1. Field of the Invention
The present invention relates to a ceramic powder suited to a thin multilayer ceramic device and a multilayer ceramic device using such a powder.
2. Description of the Related Art
A multilayer ceramic capacitor is broadly used as a compact, large capacity, high reliability electronic device. A large number are used in electrical equipment and electronic equipment. In recent years, along with the reduction in size and improvement in performance of such equipment, increasingly tough demands are being made for further reduction of size, increase of capacity, lowering of price, and improvement of reliability of such multilayer ceramic capacitors.
In recent years, demand has been rising for thin layer devices with thicknesses of dielectric layers of not more than 3 μm and further of not more than 2 μm. Along with such demands for thinner layers, it has become important to uniformly disperse the additive component in the dielectric layers into the main component. The reason is that if the additive component in the dielectric layers is not uniformly dispersed, the electrical properties, temperature properties, etc. will become nonuniform and the quality and reliability will drop. In particular, a thin layer dielectric is susceptible to the effects of nonuniform dispersion of the additive component, so uniform dispersion of the additive component is an important issue.
In the past, to cause uniform dispersion of the additive component, the technique of blending the barium titanate or other main component and additives using a ball mill etc., roasting the blend, and treating it by plasma has been usually used. With this past technique, however, it has not been possible to prevent the additive component particles in the dielectric layers from partially segregating. This segregation becomes a reason for occurrence of hetero phases between the dielectric layers. Further, these hetero phases become causes lowering the breakdown voltage of electronic devices and reducing the load life.
The size of the hetero phases caused in the dielectric layers is, in the large cases, about 1 to 2 μm. In a multilayer electronic device of a thickness of the dielectric layers greater than 5 μm, this does not become a major problem, but in the thin layer devices recently demanded, in particular, multilayer electronic devices having a thickness of the dielectric layers of not more than 3 μm or further of not more than 2 μm, the hetero phases generated have a large effect and it is not possible to secure the desired quality and reliability.
Further, as a method of suppressing occurrence of hetero phases, the method of reducing the fineness of the additives, for example, the method of forming a plurality of additives into a compound in advance by heating and then finely pulverizing the compound has been proposed. Additive component particles however tend to agglutinate more easily the smaller the particle size. Therefore, with the technique of reducing the particle sizes of the additives, it was not possible to sufficiently achieve the object of improving the dispersibility in the main components and preventing the occurrence of segregation and hetero phases.
Further, Japanese Unexamined Patent Publication (Koukai) No. 11-335176 studied the distribution of the particle sizes of the ceramic main component particles and disclosed a ceramic powder using ceramic main component particles having a predetermined distribution of particle size. That is, this publication discloses a ceramic powder having a median value of ceramic main component particles of not more than 5 μm and having a number of main component particles having a median value of not more than ⅕ of that of 10% to 30% of the overall number of main component particles.
With this conventional art, however, it was not possible to provide a multilayer ceramic electronic device having the recently demanded thickness of the dielectric layers (thickness between internal electrodes) of not more than 3 μm and further of not more than 2 μm, provided with a high breakdown voltage, long life, and high reliability.